Query multiple tables at once?

SELECT * 
FROM wp_posts, wp_2_posts, wp_3_posts 
WHERE 
    wp_posts.post_content LIKE '%foo' OR
    wp_2_posts.post_content LIKE '%foo' OR
    wp_3_posts.post_content LIKE '%foo';

should do the trick.

(Note that in Multisite, there shouldn’t be a wp_1_posts — your root site uses wp_posts etc.)